I am getting the same error. I thought I had done this before, but it may have been a text/csv data source versus xls. You may be able to use an alternate method to achieve the results you need. Look at using a derived column transformation to pad 0's back on to that colum using stuff or some other type of function since you know the data type of the column. Not the prettiest, but if no one else knows how to resolve the issue it may be your best course of action.

For example something like right('00000' + zipcode field, 5)